Process for the preparation of sulfate pulp digestion white liquor

ABSTRACT

A PROCESS FOR GENERATING A SPENT DIGESTION LIQUOR FROM A SULFATE PULP MAKING PROCESS BY ADDING A SUPPLEMENTAL LIQUOR, AND PARTICULARLY TO A PROCESS FOR EASILY CONTOLLING THE LIQUID COMPOSITION AT ANY DESIRED LEVEL BY USING A SUPPLEMENTARY LIQUOR CONTAINING AS THE MAIN INGREDIENT NASH IN PLACE OF NA2SO4.

July 18, 1972 KATSUO WATANABE PROCESS FOR THE PREPARATION OF SULFATE PULP DIGESTION WHITE LIQUOR Filed NOV. 26, 1969 g C 15 Q o 10 4o X(%) INVENTOR KATSUO 25w 11', $0 daso/z WATA NABE ATTORNEY United States Patent US. Cl. 162-45 4 Claims ABSTRACT OF THE DISCLOSURE A process for regenerating a spent digestion liquor from a sulfate pulp making process by adding a supplemental liquor, and particularly to a process for easily controlling the liquid composition at any desired level by using a supplementary liquor containing as the main ingredient NaSH in place of Na SO BACKGROUND OF THE INVENTION The present invention relates to a process for regenerating a spent digestion liquor from a sulfate pulp making process by adding a supplemental liquor, and particularly to a process for easily controlling the liquid composition at any desired level.

In the prior art, it is regarded as impossible or very diflicult to control a digestion white liquor composition, particularly to control the active alkali and the sulfidity in the liquor.

In a conventional sulfate pulp method (KP method) which has been developed by the progress of waste liquidrecovering technique, starting raw material, wood chip, is steamed under a high temperature and pressure with a digestion liquid containing NaOH and Na S at a definite ratio, whereby lignin is effectively eluted, and it is wellknown that the process has been developed by such a theory that improvements in yield and quality of pulp product can be effectively attained by the promotion of delignin reaction due to the reaction of Na s and NaSH in the alkaline solution with lignin, and at the same time by the restriction of excess reaction of alkali with cellulose.

The art for recovering digestion liquid, and particularly the art for controlling the composition (mainly the ratio of active alkali to sulfidity) of a digesting white liquid as well as the art of pulp digestion is very important in sulphate process (KP) pulp industry, and the most difficult problem for digestion liquid-recovering operation (regeneration of digestion liquid) in this -field is the control of variation of the sulfidity and the active alkali content caused by fluctuating amounts of spent Na and S contents in the digesting liquid-recovering processes.

That is, the spent Na-content in the digestion liquid used for pulp digestion by a sulphate method (KP) depends mainly on such mechanical factors as washing of pulp, washing of lime mortar in causticizing process, clarification of green liquid and recovering in a boiler, and the mechanical liquid losses such as a small amount of black liquid accompanied by pulp in the pulp-washing process or a small amount of the white liquid in the causticizing process decreases the whole chemicals in the digestion liquid at a definite ratio. Thus, the ratio of S- to Na-content is not so varied.

Of course, in order to prevent such mechanical liquid losses, many processes have been developed, that is, the sludge-washing liquid in the green liquid clarification process and the lime mortar-washing liquid in the causticizing process are recovered as a waste liquor, and the resulting waste liquor is returned to the system for dis- 3,677,885 Patented July 18, 1972 solving a melt from a recovering boiler, and further the scattering losses of Na SO and other Na-containing material in the recovering boiler are collected by an electric precipitator or the like, and thus, the liquid loss has been decreased a minimum extent (recovering rate in the prior art is between 92-95%).

On the other hand, the spent S-content additionally occurs through mechanisms other than the mechanical S-loss accompanied with the spent Na-content. Thus volatile S-compounds such as H 5, S0 R-SH, CH SCCH etc. are lost in the steps of condensation of the black liquid, recovering in the boiler or dissolution of a melt and S-compounds are also lost in the waste gas. Thus, the recovering ratio of 'Nato S-content is varied, and the chemicals balance in the recovering process has to be made something complex.

As the supplemental amount of Na for the recovered waste liquid has to correspond to the loss of Na- -conte'nt, and S-content to be supplied has to correspond to the loss of S-content, the ratio of the spent S- content to Na-content, when -Na SO is employed as the supplemental chemical, should be made to be balanced with the ratio of Nato S-content in Na SO Considering the relationship between the active alkali and the sulfidity in the digesting white liquid at plant operation, it is apparent that the sulfidity of digesting liquid or digesting white liquid is determined by the recovered chemicals ratio.

That is, when the recovering rate of the Na-content is maintained constant, the sulfidity of white liquid becomes lower if the recovering rate of the SPcontent is lower and on the contrary, if the recovering rate of the S-content is higher, the sulfidity of white liquid becomes higher. Most part of the Na-content loss in plant operation is caused by each washing process, and said loss mainly evolved due to mechanical factors, is always accompanied by the definite ratio of spent S-con-tent. On the other hand, as the S-conteut is additionally lost outside of the system in the form of volatile S-compounds in the digesting, washing, condensating or black liquid-burning processes, the recovering rate of the Na-content becomes inevitably higher (92-95%), than that of S, and the sulfidity in plant operation tends to be lowered, thus, addition of elemental S being often required in order to maintain a definite level of sulfidity in the prior art. (Refer to paragraph 3 3.5 of sulfidity described in the Handbook of Paper and Pulp-Making Technology, ed. by the Association for Paper and Pulp Technology in Japan, vol. 3, pp. 52-53 involved in chapter, Kraft Pulp and Non-Wood Pulp.)

Further, as is explained herein above, recently total quantity of the chemical liquid loss becomes lower and recovering rate of Na-content is raised in the recycling system, thus, the quantity of Na SO to be supplied is inevitably reduced, and as the result thereof, the S-content within the system is also reduced, thereby further lowering the sulfidity.

For the regeneration and the preparation of a digesting liquid in a sulphate pulp plant, it is most important to control the Naand S-content recovering rates in the digestion liquid-recovering process, and furthermore the recovering rate of Whole chemical liquid is also to be balanced with the S-content recovering rate, and these factors make it very diflicult to control and maintain the composition of digesting liquid and the preparation thereof. In particular, of recent, the recovering rate of Nacontent has been remarkably improved (92-95%), and the difference between said rate and that of S-content tends to be more and more important. In such a situation, maintenance of active alkali and sulfidity of the digesting liquid by use of Na SO as the supplying chemical is becoming a more and more difficult problem.

3 SUMMARY OF THE INVENTION It is an object of the present invention to eliminate difficulties due to the use of NaSO in the, prior art, by using NaSH partially or totally in place of Na SO for example in combination with Na S, NaOH or Na SO thereby making it possible to prepare a digesting liquor of any desired composition from a waste digestion liquor.

The inventor of the present invention, for the purpose of overcoming the disadvantages of prior art as explained above, has tried the employment of a digestion liquid containing essentially NaSH as the more elfective supplying chemical in place of Na SO said digestion liquid being produced from NaOH waste liquid obtained from petroleum plants according to the Japanese Pats. Nos. 401,527, 404,799 and 418,472. After studying the prop erties of NaSH as the supplying liquor and the process therefor, it has been found that NaSH has excellent characteristics which will be explained hereinafter in detail as compared with the conventional supplying liquids like Na SO or Na S, and that it is the supplying liquid most effective for controlling the regeneration and preparation of active alkali and sulfidity.

As explained hereinbefore, the composition of the recovered waste liquid in a sulphate method is determined by the chemicals lost in the digesting and liquid-recovering steps. It is well-known by those skilled in the art that the loss of these chemicals, particularly the recovering rates of Naand .S-content and the difference therebetween in plant operations are, of course, different depending on the equipments and the operating conditions of each plant. Even in the same plant, there is some difference in every operating unit, thus making it diflicult to maintain and control the liquid composition.

The inventor has tried to manufacture various recovered liquid (recovered white liquid) for reference use without any addition of supplemental chemicals, calculating from the recovering rates of Naand S-content in the plants, and has tried to use Na s (made from Na SO in a recovering boiler) and NaSH for supplying the said chemicals depending on the extent of active alkali and sulfidity. As the result, it has been found that the quantities of the supplemental chemicals and the ratio of chemicals in the supplemental liquids (chemical-containing ratio) are important factors for the regeneration of the spent liquid. That is, it is understood and confirmed that there is a relationship, as illustrated in FIG. 1, between active akali and supplement ratio of the S-content, which is a most important and very difficult factor for the preparation of digesting white liquid, when Na S(Na SO is employed as the supplemental chemical and also when NaSH is supplied for the regeneration of spent chemicals.

That is, using Na SO as the supplemental chemical, the preferable additive chemical composition (the composition of the spent chemical) was studied and examined based on the active alkali and the S-content (Na s-content) which have most important influences on digestion, and as the result thereof, it was found that there is the relationship as shown by the following equation, after studying the quantity of spent chemical of the digesting liquid based on the standard active alkali and the S-content in the digestion liquid (digestion white liquid):

y6/2OX wherein y is a quantity by percent of the supplied Nacontent (calculated as Na O) against the active alkali in the digesting white liquid, and X is a quantity by percent of the supplied S-content (calculated as Na O) against the S-content (Na s-content) in the digesting white liquid. Accordingly, it is understood that the spent ratio (percent) of the active alkali content based on the spent ratio (percent) of the .S-content in the whole amount of chemicals spent in each process is most preferably to be controlled to 30% based on the S-content. When the S-content is more than that, shortage of the active alkali (Nacontent) occurs, and thus NaOH is to be supplied together, and when it is less then that, shortage of the S- 4 content occurs, and as it is impossible to supply it with Na S (Na SOaQ, the supplement of the S-content by elemental S should be considered.

On the contrary, when NaSH is used as the supplemental chemical according to the present invention, the relationship between the ratio (percent) of spent S-content (Na S) and the ratio (percent) of spent active alkali is represented as follows, based upon the active alkali and the S-content in the digesting liquid (digesting white liquid):

As apparent from the Equation 2, it is found that the spent ratio (percent) of the active alkali content (calculated as Na O) when NaSH is used as the supplemental chemical is balanced to 15% of the spent ratio (percent) of the S-content and accordingly even when the S-content is more than that, the complete supplement can be attained by using it together with NaOH or Na s.

As is clearly understood from the above explanation, it was found that in a conventional method wherein (Na 'SO or Na S is used, the composition required for the supplemental chemical, particularly the ratio of active alkali content to S-content (spent ratio (percent) of active alkali to spent ratio (percent) of S-content) which is an important factor for digestion is to be more than 30% and said lower limit 30% can be lowered down to 15% by using NaSI-I as the additive chemical according to the present invention. Thus, the efiective range for the liquid has been greatly increased.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows characteristic curves of Na s and NaSH illustrated by spent ratio (percent) of Na-content (calculated as Na O) marked on the vertical axis and spent ratio (percent) of S-content (calculated as Na O) marked on the horizontal axis in a waste sulfate pulp digesting liquor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, if Na S is used as the supplemental chemical, it can regenerate a waste liquid having a recovering rate of Na-content that is no higher than 89.5% (spent ratio 10.5%) when the waste liquid has a recovering rate of 65% of S-content (spent ratio 35%). This maximum recovering rate of Na-content that a waste liquid can have in order to be fully regenerated by Na s is illustrated at point a for a waste liquid having an S-content recovering rate of 65%. In this case, the spent ratio (percent) of Na-content to spent ratio (percent) of S-content is about 30% (10.5%-:-35% =30%) based on that of the S-content.

On the other hand, when NaSH is used for the sup plemental chemical for a waste liquid having a recovering rate of 65% of S-content (spent ratio 35%), it can regenerate waste liquids having a recovering ratio (percent) of Na-content up to 94.75% (spent ratio of Nacontent 5.25%). Thus, by using NaSH it is possible to regenerate a wide range of recovered waste liquids having up to 94.75% of Na-recovering rate, and the NaSH is most suitable for regenerating recovered waste liquids of :high recovering rate, particularly of high Na-recovering rate. The maximum recovering rate for Na-content that a Waste liquid can have in order to be fully regenerated by NaSH is illustrated at point b for a waste liquid having an S-content recovering rate of 65%. In this case, the spent ratio of Na-content to the S-content is at about 15 Accordingly, when the S-recovering rate is 65% (spent ratio 35%) Na s or Na SO' can be used as a supplementary liquid when the Na-recovering rate is no more than 89.5%, but when the recovering rate of Na-content is more than that, shortage of the S-content occurs with these supplemental chemicals, and controlling or maintaining the digesting liquid (digesting white liquid) composition becomes impossible by said chemicals.

According to the present invention, the use of NaSH makes it possible to regenerate and maintain the digesting liquid composition (digesting white liquid) up to even when the Na-recovering rate is as high as 94.74%. At the present time, the recovering rate of the liquid, particularly of Na-content is becoming remarkably improved (9295%). By means of NaSH, the control of the active alkali and the sulfidity, which are regarded as the most diflicult matter in the kraft pulp fields, can be strictly and easily attained. This is a great advance in the kraft pulp field.

For understanding the present invention, more in detail, examples of the present invention will be explained hereinafter, but the present invention is not limited to the following examples.

TABLE 6.COMPOSITIONS OF REGENERATED WHITE LIQUOR PREPARED BY ADDING VARIOUS SUPPLE- MENTARY LIQUIDS TO COMPOSITION C 5 NaOH, g./l 76. 80 71. 67 84.00 83. 99 NazS, g./l.. 36.00 36. 01 36. 36. 00 NaiCO g./l 8. 33 8. 12 8.34 8. 12 Sulfidity,percent 80. 90 33. 48 30. 00 30. 00 Active alkali, g./L 112. 80 107. 68 120. 00 120. 00 Efiective alkali, g./l 94. 80 89. 68 102. 00 102. 00

Example l.Preparation of regenerated liquor compositions from a waste digestion liquor of composition A TABLE 1.-DIGESTING WHITE LIQUID AND RECOVERING LIQUID Digesting Recovered Recovered Recovered starting liquid liquid A liquid B liquid C Comp. N940 Comp. NazO Comp. NazO Comp. N320 30. 0 20. 19. 45 24. 7 Active alkali, g./l 120.0 117.0 111. 0 102.0 Efiective alkali, g./l 102. 0 104. 5 100. 2 89. 4

Liquid pulp digestion is carried out using a digesting white liquid (hereinafter called digesting starting liquid) as shown in Table 1. Recovered waste liquors of the compositions of points A, B and C on FIG. 1 and Table I]. are tested. These samples are studied for the effect obtained by adding difierent supplementary liquors, particularly the infiuence on the control of active alkali and sulfidity. In one study, a liquid having the composition shown in Table 2 and consisting essentially of NaSH and containing N-agS, Na CO and other Na salts is added to recovered waste liquids of compositions A, B and C. In a second study, a liquid having the composition shown in Table 3 and containing Na s as the essential component and Na CO and other Na salts is added to recovered waste liquids of compositions A, B and C. The results obtained for the regenerated liquids A, B and C are illustrated in Tables 4, 5 and 6, respectively.

Compound, Na(Naz0) g.[1. gJl.

TABLE 3.COMPOSITION OF SUPPLEMENTARY LIQUID CONTAINING Nags WITHOUT NaSH (Nags LIQUOR) Compound, Na(Na: g. [1. g.[l.

Nags

222. 60 NB CO:

TABLE 4.-COMPOSITIONS OF REGENERATED WHITE LIQUOR PREPARED BY ADDING THE SUPPLEMEN- TARY LIQUIDS OF TABLES 2 AND 3 TO COMPOSITION A Active alkali, g./l

Efiective alkali, g./1 I 110.70

Sulfidity, percent..- Active alkali, g./l Efiective alkali, g./l

trates the composition of a regenerated liquid obtained by adding 6.62 parts of the Na S solution of Table 3 to parts of waste liquid composition A. Waste liq uid composition A has 11.7 g./l. of spent sulfur as disclosed in Table 1.

As is clearly understood from a comparison of the starting digesting liquid composition with the regenerated liquor obtained by adding the NaSH liquor, 2.395 g./l. of excess active alkali and the regeneration of Na- S content can be obtained When NaSH is used. However, the regeneration of sulfidity for composition A cannot ,be obtained when NaSH is used without resulting in an excess alkali content as compared with the starting digesting liquor composition. Likewise, a comparison of the starting digesting liquor composition with the regenerated liquid obtained by adding the Na S shows that 8.70 g./l. of excess active alkali and regeneration of Na S content is obtained when the Na S liquor is used. However, as with the NaSH liquor, regeneration of sulfidity cannot be obtained when the Na S solution is used without resulting in an excess alkali content.

Thus, regeneration of spent Na (active alkali) of recovered liquor A of Table l by using the NaSH solution of Table 2 resulted in 5.98 g./l. of sulfur insufiiciency, and regeneration by the Na s solution of Table 3 resulted in 8.7 g./l. of sulfur insufficiency (calculated as Na O).

The above description can be easily understood from the relation between Na content, S content, spent ratio and supplemental chemicals of FIG. 1. As is seen from the above explanation, it is impossible to prepare completely regenerated liquor by using NaSH solution or Na S solution. Further, it is more effective to use NaSH than Na s (which is obtained by adding Na SO to a recovering boiler), for the regeneration of active alkali and sulfidity of such a wasted liquor from KP pulp plant as requiring a supplemental liquor having less than 15% of spent Na ratio based on spent S ratio.

Example 2.Preparation of regenerated liquor compositions from a waste digestion liquor of composition B Table 5 illustrates the regenerated compositions obtained by adding different supplementary liquors to a recovered liquid of composition B. Composition B has a 92.5% active alkali recovering rate (7.5% spent alkali) and a 60.0% sulfur recovering rate (40% spent S). Experimental results are obtained by adding the NaSH solution of Table 2 and the NagS solution of Table 3 into 7 a wasted liquor having the composition B of FIG. 1. Table illustrates the results obtained by adding 8.15 parts of Na s solution of Table 3 into 100 parts of wasted liquor having the composition B of FIG. 1. Composition B of FIG. 1 has 14.40 g./l. of sulfur insufficiency z l- As is clearly understood from the comparison with the initial digestion liquor composition, 5.40 g./l. of excess active alkali was observed and the regeneration of sulfidity could not be attained by using the Na s solution. In other words, supplementing spent sulfur by using Na S resulted in sulfur insufliciency, and also in lowering the sulfidity. Thus, it is a quite serious and diflicult problem to control the sulfidity of the liquor, due to the technical difliculty in the sulfur supplying system, and the recent remarkable improvement in Na recovering techniques.

Table 5-2 illustrates one of the experimental results obtained by adding the NaSH solution of Table 2 into a wasted recovered liquor of composition B of Table I having a sulfur insufiiciency of 14.4 g./l. The composition of Table 52 is obtained by adding 4.276 parts of the NaSH solution of Table 2 into 100 parts of a wasted recovered liquor of composition B.

As is clearly understood from a comparison with the standard digestion liquor of Table 1, use of the NaSH liquor of Table 2 results in an insufiiciency of active alkali and improvement in the sulfidity level. This is quite contrary to the case of the Na S solution. Further, excess sulfur content (Na s) is obtained by adding NaSH solution into a liquor having Nainsufficient wasted recovered liquor composition B, thereby improving the sulfidity. This is easily understood from the relations between the spent ratio of Na and S in the supplemental liquor of FIG. 1.

It has been found that active alkali content and sulfidity level can be easily controlled by using a mixture of the NaSH solution of Table 2 and the Na S solution of Table 3, or a mixture of the NaSH solution and an NaOH solution. Tables 5-3 and 5-4 show the examples thereof.

It is apparent from the experimental results of Table 5-1-2-3-4 and the relation of spent ratio of Na and S in FIG. 1, that it is impossible to regenerate the sulfidity and active alkali of kraft pulp wasted recovered liquor requiring a supplemental liquor having 15-30% of Na spent ratio based on the S spent ratio, only by using N21 SO (Na S). However, this problem can be easily solved by a process according to the present invention, wherein NaSH, Na SO (Na s), and NaOH are used in combination or respectively to regenerate active alkali and sulfidity of liquor.

Further, according to the present invention, even in a KP plant equipped with a highly improved Na recovering system, wherein the prior art technique of by adding Na SO to a recovering boiler cannot be used, it is quite easy to regenerate and control the active alkali and sulfidity of a recovered waste liquor. This is one of the remarkable advantages of the present invention in view point of the recent tendency of further improvement in the Na recovering system.

Example 3.Preparation of regenerated liquor compositions from a waste digestion liquor of composition C Table 6 illustrates liquor composition of C point in FIG. 1 (85.0% of active alkali recovering rate and 70.0% of sulfur recovering rate) and the experimental results obtained by adding NaSH solution and Na S solution as represented by Table 2 and Table 3 respectively into a liquor having the composition of point C of Table 1. Table 6--1 is the experimental result obtained by adding 6.11 parts Na s solution represented by Table 3 into 100 parts of recovered liquor having 10.8 g./l. of sulfur insufficiency of composition C in Table 1. According to the results obtained, increased sufidity due to insufiicient active alkali content was observed while Na s was regenerated compared with the initial digestion liquor of Table 1. This is based on the ingredient ratio of active alkali 8 and Na 'S of the Na S solution used for the supplemental liquor and illustrates that Na s solution has insuflicient alkali for the regeneration of required composition of the recovered waste liquor C.

The present invention has succeeded in regenerating a liquor of the composition similar to the initial digestion liquor of Table l by using Na S solution together with NaOH. Thus obtained composition is shown in Table 6-3.

Table 6-2 illustrates a liquor composition obtained by adding 6.11 parts of NaSH solution of Table 2 into parts of recovered wasted liquor having the composition C of Table 1 and 10.8 g./l. of sulfur insufl-iciency. As clearly understood from the said composition compared with the initial digestion liquor, complete regeneration of sulfur content resulted in active alkali insufliciency thereby increasing the sulfidity thereof, as in case of using Na S solution of Table 6-1. This is based on the insufficiency of active alkali in NaSH solution when used for the supplemental liquor, as illustrated in Table 6-1. Accordingly We have succeeded in obtaining the regenerated liquor of strictly controlled sulfidity and active alkali content as shown in Table 6-4 by using NaSH in combination with NaOH.

As summarized from the explanation hereinbefore, active alkali content of the waste recovered liquor of the composition C in FIG. 1 cannot be regenerated by only adding NaSH solution or Na S solution respectively for the supplement of sulfur insufiiciency, and can be regenerated only by using them together with NaOH.

Referring to the experimental results described in Table 6-1-2-3-4 and the relations between spent Na and S ratios of FIG. 1, it is apparent that complete regeneration of active alkali and sulfidity of a waste recovered liquor from kraft pulp plant requiring more than 30% of spent Na ratio based on the spent S ratio cannot be attained only by adding Na SO (Na s) or NaSH solution respectively and can be attained by adding together with NaOH. According to the process of the present invention wherein NaSH is used in combination with NaOH, remarkably improved results can be obtained compared with prior art wherein Na SO- is added to a recovering boiler.

Example 4.-Pulp digestion Digestion liquors of the compositions exemplified in Table 7 by using liquors exemplified in Table 5-3-4, Table 6-3-4 etc., said liquors being completely regenerated in active alkali and sulfidity by using as the supplemental liquors Na S solution of Table 2 and NaSH solution of Table 3, is prepared and tested in order to study the infiuence of the liquor prepared by Na s solution and NaSH solution on pulp digestion, particularly influence on fines yield and Roe number and the like, of the digested pulp, and found that there are no remarkable difference in pulp quality obtained as seen in Table 8 and Table 9 and NaSlI-I solution can be used conveniently for the purpose of efiective digestion liquor.

TABLE 7.DIGESTION LIQUOR COMPOSITION FOR TEST PURPOSE Reference 1 2 sulfidity, percent 2& 00 27. 91 28. 13 Active alkali, gJl 115. 00 115. 13 114. 82 Efiective alkali, g./1 99. 5 98. 5O 98. 68

TABLE 8.FINES YIELD OBSERVED In the foregoing explanation, the diiference of Na SO (N1 8) and NaSH in relation to the capacity as supplemental liquors and as pulp digestion liquor were discussed in detail based on many experimental results. Thus, eifectiveness of Na s and NaSH when used for supplemental liquor are summarized as follows. In the prior art a waste recovered liquor having more than 30% of spent Na ratio (percent) based on the spent S ratio (percent) can be regenerated by adding Na SO or Na s respectively or in combination with NaOH. However, a waste recovered liquor having less than 30% of spent Na ratio (percent) based on the spent S ratio (percent) can be regenerated only by using elemental sulfur or NaSH together therewith for the regeneration of the active alkali and the sulfidity. Accordingly, they are not preferable chemicals for the supplemental liquors. Further, addition of elemental sulfur into the liquor causes formation of Na Sx by reacting with Na S, said Na sx having undesirable effects on the pulp digestion and producing different action on pulp digestion from KP digestion. This is a widely known fact to those skilled in the art. Accordingly the process according to the present invention has employed NaSH having no undesirable efiects and considerable sulfur content. This is a great advantage for the prior art.

According to the present invention, NaSH does not form Na Sx and can regenerate easily and severely the sulfidity and the active alkali of the wasted recovered liquor having more than 15% of spent Na ratio (percent) based on the spent S ratio (percent), by using NaSH itself, Na S and NaOH respectively or in combination therewith. This a remarkable advantage in viewpoint of technical and economical points for the prior art wherein Na SO (Na S) is used. Furthermore, from the viewpoint of operation in KP plant, addition of Na SO to a recovery boiler can be eliminated or greatly decreased by adding NaSH directly after the causticization step, thereby decreasing boiler loading, increasing boiler capacity, increasing heat evolved in the boiler without any absorbing of reaction heat from Na SO to Na S and resulting in increased steam amount evolved, and quality and yield of pulp obtained are easily improved by severely controlling the sulfidity, active alkali, etc.

I claim:

1. In a process for the preparation of regenerated sulfate pulp digestion white liquor from a waste digestion liquor which has been recovered from an original white liquor used in a digestion process the improvement which comprises adding a supplementary liquor containing NaSH to the waste recovered digestion liquor to control the sulfidity, active alkali and effective alkali in the regenerated white liquor.

2. The process according to claim 1 wherein in the waste recovered digestion liquor the amount of sodium spent based on the total amount of Na present in the original white liquor (calculated as Na 0) is more than 15% of the amount of sulfur spent based on the total amount of sulfur present in the original white liquor (calculated as Na O).

3. The process according to claim 1 wherein the supplementary liquor also contains at least one of the compounds NaOH or Na S and wherein in the waste recovered digestion liquor the amount of sodium spent based on the total amount of Na present in the original white liquor (calculated as Na 0) is more than 30% of the amount of sulfur spent based on the total amount of sulfur present in the original white liquor (calculated as N320).

4. The process according to claim 1 wherein the supplementary liquor also contains at least one of the compounds NaOH or Na S and wherein in the waste recovered digestion liquor the amount of sodium spent based on the total amount of Na present in the original white liquor (calculated as Na O) is from 15 to 30% of the amount of sulfur spent based on the total amount of sulfur present in the original white liquor (calculated as Na 'O).

References Cited UNITED STATES PATENTS 1,689,534 10/1928 Richter 162--30 3,514,371 5/1970 Watanabe 16229X S. LEON BASHORE, Primary Examiner A. A. DANDREA, JR., Assistant Examiner U.S. Cl. X.R. l62-82 

